1. Field of the Invention:
This invention relates to a novel and improved anti-skid control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from skidding during braking operation of the motor vehicle.
2. Description of the Prior Art:
Generally, with an anti-skid control system for motor vehicles, anti-skid control is effected by means of microcomputers such that hold valves and decay valves comprising electromagentic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration +Vw, -Vw and brake hydraulic pressure Pw are varied during the operation of the conventional anti-skid control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves.
When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, and the brake hydraulic pressure Pw is not increased; and when the brake equipment is being operated, the brake hydraulic pressure Pw is increased at time t1 so that the wheel speed Vw is decreased. A reference wheel speed Vt is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, reference wheel speed Vt is set up so that the deceleration (negative acceleration) -Vw of the wheel reaches a predetermined threshold level, say -1G at time t2, the reference wheel speed Vt is thereafter made to linearly decrease with a deceleration gradient of -1G. At time t3 when the deceleration -Vw of the wheel reaches a predetermined maximum value -Gmax, the hold signal HS is interrupted so that the hold valves are closed, thus holding the brake hydraulic pressure Pw.
Because of the brake hydraulic pressure Pw being held, the wheel speed Vw is further decreased. At time t4, the wheel speed Vw and the reference wheel speed Vt become equal to each other, and the decay signal DS is generated, by which the decay valves are opened so that reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the wheel speed Vw is changed from deceleration to acceleration, at time t5 when a low peak VL of the wheel speed Vw occurs. Either at the time t5 or at time t6 when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak speed VL by 15% of the difference A between the wheel speed Va occurring at the time t4 when the reduction of the brake hydraulic pressure is started (Vb=VL+0.15A) and the low-peak speed VL, the decay signal DS is interrupted, and as a result the decay valves are closed so that the reduction of the brake hydraulic pressure Pw is stopped and thus the brake hydraulic pressure is held. The wheel speed Vw is further increased and a high peak VH thereof is reached at time t7; thereupon, the brake hydraulic pressure Pw is again increased. In this case, the buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal is turned on and off mincingly so that the brake hydraulic pressure Pw is caused to gradually build up. In this way, the wheel speed Vw is decreased, and at time t8, the mode for reduction of the brake hydraulic pressure occurs again. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
When the motor vehicle is running on a rough road, it frequently happens that the wheels thereof are caused to float in the air. In such a state, if the brake equipment of the vehicle is operated through the aforementioned anti-skid control operation, then the wheels will tend to be subjected to early-locking, and thus when the wheels are caused to land on the road again while being locked, changes in the wheel speeds turn out to be different from those which occur when the vehicle is running on a normal road. More specifically, when the vehicle is running on a rough road, the cycle of the anti-skid control turns out to be faster than when the vehicle is running on a normal road, and the amplitude of the wheel speed Vw also becomes larger. Disadvantageously, therefore, in the case where even when the vehicle is running on a rough road the reference wheel speed Vt from which a point of time when reduction of the brake hydraulic pressure is to be started, is set up on the basis of a standard similar to that that is used when the vehicle is running on a normal road, an pressure reduction starting point will be reached soon after buildup of the brake hydraulic pressure was started at a high peak of the wheel speed, so that the brake hydraulic pressure will be prevented from building up, as a result of which the braking distance will be increased.